Optical element having antireflection film

An optical element having an antireflection film comprising a plastic substrate having a basic layer and an antireflection film of a λ/4-λ/2-λ/4 type provided thereon, wherein

• the basic layer and the antireflection film are of a eight layer construction having a basic layer of metallic niobium, equivalent film λ/4 (1st to 3rd layers), equivalent film λ/2 (4th to 6th layers), and equivalent film λ/4 (7th layer), and wherein
• the 1st layer is a silicon dioxide layer having a refractive index of from 1.43 to 1.47,
• the 2nd layer is a high refraction layer having a refractive index of from 2.04 to 2.37,
• the 3rd layer is a silicon dioxide layer having a refractive index of from 1.43 to 1.47,
• the 4th layer is a high-refraction layer having a refractive index of from 2.04 to 2.37,
• the 5th layer is a silicon dioxide layer having a refractive index of from 1.43 to 1.47,
• the 6th layer is a high-refraction layer having a refractive index of from 2.04 to 2.37,
• the 7th layer is a silicon dioxide layer having a refractive index of from 1.43 to 1.47,
• the resultant refractive index of λ/4 (1st to 3rd layers) is from 1.65 to 1.80,
• the resultant refractive index of λ/2 (4th to 6th layers) is from 1.85 to 2.25,
• the high-refraction layers (2nd, 4th and 6th layers) are each constructed of at least one metal oxide selected from titanium oxide, niobium oxide and tantalum oxide.

An optical element comprising

• a plastic substrate,
• a cured film obtained by curing a coating composition which comprises metal oxide colloid particles and an organosilicon compound on said plastic substrate,
• a basic layer of silicon dioxide in direct contact with said cured film,
• an antireflection film formed on direct contact with said basic layer, which comprises an equivalent film λ/2 containing at least three layers and having a refractive index of from 1.80 to 2.40 wherein the even-numbered layer(s) of the equivalent film is a silicon dioxide layer or a layer made of silicon dioxide as a main component and wherein the antireflection film also contains at least one equivalent film λ/4 on each side of the equivalent film λ/2.